Oh Tish, that sounds soooo much like my
DS.
First thing I would reccomend is don't keep trying to get him to sleep if it doesn't work after 15 minutes or so. Just give it up, go do something else for a while and wait till your next window of opportunity. Seriously - trying for hours to get him to sleep when he's only going down for 30 minutes is TORTURE!
Second, for us, after 6 months or so the whole 'put them down at the first sign of tiredness' didn't work any more. My boy didn't care if he NEEDED sleep, he would only go down once he was sooooo exhausted he just couldn't stay awake anymore and so was WILLING to sleep.
Few other things to consider.
What time is bedtime? Sometimes bringing it forward can help.
How long is your bedtime routine. Sometimes they need quite a long wind down.
You're not alone (as cold comfort as that is!) and yes, one day it will get better...